The invention relates to a variable equalizer including an active circuit having a variable transfer characteristic for compensating deviation of a transmission characteristic.
In wire transmission systems using a coaxial cable or a cable pair, the transmission characteristic of the cable varies depending upon the length of the path in the transmission medium and the ambient temperature. A Bode type variable equalizer often is used to compensate for the variation of the transmission characteristic because such an equalizer possesses a unique property in that its equalization shape does not change as the amount of equalization is varied. In mathematical terms, the equalization shape V(.omega.) closely approximates an ideal relationship which is expressed as EQU V(.omega.)=.rho.F(.omega.) dB
for -1&lt;.rho.&lt;+1, where V(.omega.) and F(.omega.) are equalization shapes in decibels and .rho. is a control parameter which determines the amount and sign of the equalization.
H. W. Bode has shown that a practical transform function which can be represented by an expression ##EQU1## produces a response characteristic that is very close to the ideal relationship. In this transform function, x is a control parameter for controlling the sign and magnitude of the equalization. The function F(s) is a transfer function of the frequency characteristic produced for maximum equalization, i.e., when the parameter x=.infin..
Bode equalizers generally are passive circuits and typically require inductors. As a result, Bode type variable equalizers are not fabricated in integrated circuit form.
Several active variable equalizers without inductors have been disclosed in the prior art. Some of these prior art active variable equalizers, such as those disclosed by F. Brglez in U.S. Pat. No. 3,921,105, are sensitive to variation of both amplifier gain and passive component values because they rely upon positive feedback and/or signal subtraction to achieve equalization. Other prior art active equalizers, such as those disclosed by Y. Takasaki et al. in U.S. Pat. No. 4,004,253, employ overlapping feedforward and feedback loops requiring a pair of closely-matched shaping networks and a wide-ranging control function or a pair of closely-matched wide-ranging control functions.
It is an object to provide an active variable equalizer not requiring an inductor.
It is another object to provide an active variable equalizer having a transfer characteristic such that the equalization shape does not change as the amount of equalization is varied.